RCS provides far better support for group chats, many small feature improvements and end-to-end encryption (E2EE) via Messaging Layer Security (MLS). E2EE for talking to GMS Android and iOS users without them needing to install another app is the main reason we care about it, but far better group chat support matters too. iPhone users very often don't want to have SMS/MMS users in their group chats.

All carriers I've tried in my country have even disabled MMS entirely. They're not running RCS servers either, though, everyone is on WhatsApp anyway.

In theory carriers could increase the max resolution for MMS now that 3G is essentially dead, but I doubt they will now the slow move to RCS has started.

Perhaps it's different in other countries, but I've never seen anyone use MMS. (Everyone uses WhatsApp nowadays, but even before WhatsApp existed, I never saw anyone use MMS.)